1. Field of the Invention
The present invention relates to a balanced line driver circuit having a built-in BON (build-out Network). More particularly, it relates to a BON built-in type balanced line driver circuit for converting a unipolar signal to a bipolar signal and sending-out the signal to the balanced line. Further, it relates to subscriber line termination equipment on which is provided the BON built-in type balanced line driver circuit of the present invention.
2. Related Prior Art
FIGS. 1 and 2 show a first and a second conventional circuits of a BON built-in type balanced line driver circuit. More particularly, it is a driver circuit for converting the unipolar signal to the bipolar signal and sending-out the relevant bipolar signal to the balanced line. The conventional BON built-in driver circuit separates direct current for giving the reverse characteristic for the frequency characteristic of the balanced line to the sending-out signal with separating direct current.
In FIG. 1, reference numerals "1" and "2" designate NPN type transistors, the emitters of which are commonly connected and which have a switching function wherein the transistors turn ON when positive and negative unipolar signals (+PCM, -PCM) are supplied to the bases of the transistors, alternatively.
Numeral "3" is a transformer including a primary winding 31 having an intermediate tap 32 on the primary side. In the example of FIG. 1, a bias electric power supply 4 is supplied between the commonly connected emitters of the transistors 1 and 2 and the intermediate tap 32.
A secondary winding 33 of the transformer 3 is connected to a balanced line 5 via impedance circuits 8 and 9 having the same impedance characteristic (ZA=ZB). The end of the balanced line 5 is connected to a load impedance (ZL) 6.
On the above-described structure, because the transistors 1 and 2 are turned ON when the unipolar signals (+PCM, -PCM) are supplied to the bases, alternately, the corresponding bipolar signals are outputted to the secondary winding 33 of the transformer 3.
On the other hand, the balanced line 5 has such a characteristic that the attenuation amount of high frequency components becomes larger corresponding to a line length, that is, a low-pass filter characteristic. Accordingly, it is required to compensate the attenuation amount of the high frequency components by contributing the reverse characteristic of the balanced line characteristic which is the relevant low-pass filter characteristic in advance, so that pulse signals can be received with less distortion at a load side of the balanced line 5.
The impedance circuits 8 and 9 contribute the reverse characteristic of this balanced line characteristic and are required to be provided between the secondary winding 33 and the balanced line 5 in the form of pair, to keep the balance.
The conventional structure in FIG. 2 makes the same impedance circuits 8 and 9 required in the form of pair in FIG. 1 compose one impedance circuit 11 in FIG. 1. Accordingly, the structure includes an unbalanced/balanced converting transformer 10, unlike the structure of FIG. 1.
It is possible to send the bipolar signal to the balanced line 5 for keeping the balance by the unbalanced/balanced converting transformer 10, even if it is in the case where only the impedance circuit 11 for giving the reverse characteristic of the balanced line characteristic is provided. However, it is required that the impedance characteristic of the impedance circuit 11 makes 2 ZA.
And then, let us now consider about this impedance circuit 11 in detail.
FIG. 3 is a functional diagram of the conventional circuit. In the diagram, an unbalanced/balanced (U/B) converting section 71 includes the transistors 1 and 2, the transformer 3, and a section for converting the unipolar signal to the bipolar signal, in FIGS. 1 and 2.
An EQL section 72 and a BON/SW section 73 include the impedance circuits 8 and 9 or the impedance circuit 11 in FIGS. 1 and 2.
The EQL section 72 is an equalizing circuit for giving the reverse characteristic of the frequency characteristic of the line in the case where the balanced line length which is connected is longest. As shown in the diagram, the EQL section has a high-pass filter characteristic, (the less loss, the higher frequency). Further, for example, in the case where the equalizing shape is determined by the specification of the network, it is required to equal the pulse shape so as to suit the relevant specification.
FIG. 4 is one example of a pulse template for the North American DS-1 equipment provided by Bell specifications. It is required to equalize a waveform so that the equalizing shape of the waveform is within the pulse template. In the example of FIG. 4, an over shoot 80 and an under shoot 81 of the pulse is particularly required.
In FIG. 3, a BON/SW 73, having a plurality of dummy lines 731 through 733, is a circuit having the low-pass filter characteristic for connecting only one of the dummy lines 731-733 to the balanced line 5 by selectively switching the switch 734.
That is, the dummy line characteristic, having the low-pass filter characteristic, is switched and added corresponding to the balanced line length which is connected, to compensate for the dynamic range of the impedance circuits 8 and 9 and the impedance circuit 11.
For example, BON 0 (731) is a dummy line inserted in the case where the longest line length, for example, the line having the length of 500-750 feet, is connected, and the frequency characteristic is flat. The BON 0 (731) is designed so that the EQL section 72 has a equalizing characteristic for providing the reverse characteristic of the frequency characteristic of the line in the case where the length of the balanced line is longest.
The BON 250 (732) is a dummy line which is inserted in the case where the intermediate line length (250-500 feet) is connected. The BON 500 (733) is a dummy line which is inserted in the case where the shortest line length (0-250 feet) is connected.
In FIG. 3, numeral "5" is the balanced line explained in FIG. 1, having the low-pass filter characteristic corresponding to the length. "6" is a load circuit, for example, a cross connect circuit as the latter-described.
FIG. 5 is a detail diagram of the conventional circuit, which is corresponding to the structure of the second conventional circuit in FIG. 2. FIG. 6 is a diagram showing the relation between the combination of ON and OFF of each switch in FIG. 5 and the kind of BON. Further, the same or similar numerals to those shown in FIGS. 1 through 4, are shown the same reference numbers and symbols.
In FIG. 5, transformer 3 is shown with abridgment of the bias circuit. T and R are two terminals connected to the balanced line 5. The EQL section 72 is a high-pass filter constituted of the capacity C1 through C4, the resisters R1 through R3 and an inductor L1.
The BON/SW section 73 has the resisters R5 through R14, the inductors L2 through L3 and the condensers C5 through C6 as the elements constituting BON, and the switch 734 has SW1 through SW5.
The BON 0 (731), the BON 250 (732) and the BON 500 (733) include the combination of these switches SW1 through SW5. The combinations of switches are shown in the table of FIG. 6. For example, BON 0 (731) includes SW3 and SW5 in an ON state and SW1, SW2 and SW4 in an OFF state.
The conventional circuits as shown and described in FIGS. 1-6 have several problems as follows;
That is, the same impedance compensating circuits 8 and 9 (ZA, ZB) must be connected to the balanced line 5, to keep the balance on the structure of FIG. 1. Further, in the circuit of FIG. 2, although only the one compensating impedance circuit 11 is required, the unbalanced/balanced converting transformer 10 becomes necessary.
On the other hand, as the BON built-in type balanced line driver circuit having the unipolar or the bipolar signal converting circuits are provided on a low-rank group channel unit, it must have the same number of driver circuits as channels. Accordingly, a pair of the same impedance compensating circuits are required, or the unbalanced/balanced converting transformer is required in the conventional circuit. In either case, the scale of the circuit becomes larger. Thus, it interferes with reducing the size of the equipment and reducing the cost.